It is already known that some rare gene variants can cause Parkinson's. Some of these are inherited in what is called an 'autosomal dominant' way i.e. each child of a person with this type of gene change has a 50% chance of inheriting it. However, it is also known that not everyone who carries the change will go on to develop the disease. Some people appear to be protected against developing the disease and the investigators would like to understand this better. Other variants, particularly important in early onset Parkinson's are "recessive", and in this form of inheritance there is a very low risk to parents and children.